How good is the 5.3L V8 for towing ???

The reason I ask this question is that I used to own a 1999 Chevy Silverado with the 5.3L V8 and 3.73 gear ratio and it felt cluggish when towing especially when towing through the Canadian Rockies (I own a 23' Travel trailer which weighs around 6,000 lbs when loaded). i had this truck for 4 years and traded it in for a 2004 Nissan Titan with the 5.6L V8 - now this engine could pull - mountain roads were no problem - so now almost 8 years later, I've decided that I need a new truck - so I've gone and factory ordered a 2012 GMC Sierra 1500 SLE EC 4x2 with the 5.3L V8 and 6 sp transmission and 3.42 gear ratio - So how good is this engine/transmission combination for towing - I'm hoping that it is at least as good as the TItan - but hoping that it's better than the Titan

I have an 11' 5.3, 6spd and 3.42 gears. Although my truck is a 4x4. Last summer I pulled an 1800lb trailer with my 85' Harley FXR and my 06' Harley Street Glide on the trailer. I also had a BMW motorcycle in the bed. The total load weight was around 3000lb abd drove the mountains of western NC and West VA. The truck held speed going up pretty well. I wouldn't say it was sailing up the mountains but I thought it did well. I would have pushed it harder but I was watching the tranny temp start to climb. If I was going to do a lot of hauling I would add an aux tranny cooler to keep that from happening.

I own a 23' Travel trailer which weighs around 6,000 lbs when loaded.
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I've gone and factory ordered a 2012 GMC Sierra 1500 SLE EC 4x2 with the 5.3L V8 and 6 sp transmission and 3.42 gear ratio - So how good is this engine/transmission combination for towing - I'm hoping that it is at least as good as the TItan - but hoping that it's better than the Titan

I don't tow much, so I can't speak anecdotally to the combination you've chosen ... but the specs indicate (from a sheer capacity stance) that'll you'll be fine. As to whether it'll be more or less sluggish off the line than the 3.73's, unless I'm mistaken I'd think 3.42's would be more sluggish off the line as they are longer gears than 3.73's (and, thus, you'd have less torque and more fuel economy using 3.42's over 3.73's).

Welcome to the club. Nice truck...
I agree with [MENTION=50075]SurrealOne[/MENTION]. (from experience)

I had a 2009 CC 4X4 same set up for engine transmission and rear gears, the trucks have the HD towing package with external trans and oil coolers.
I use to tow a 7000# trailer with 850# of tongue weight. Yes i did pull it on flats well enough but found myself overloading the GVWR of the truck.
Up hills it was sluggish and I would take a run at them.
So if you are concerned with the GVWR take your tongue weight add full fuel load, all passengers in the vehicle, gear in the back of the truck, and the curb weight of the truck and add it all up and it must not exceed the GVWR. Add the trailer and keep this weight below GCVWR.

Let me know if you have any other questions. Please note that I now have a 2500HD with 3.73 gears! Upgraded to pull the trailer better.
The reason I mention GVWR is last year there was a blitz in my area of HWY 401 where the Ministry of Transportation had a number of Trucks pulling RV's on the side of the road weighing the trucks. I wasn't one of them!

I failed to note the extended cab (I missed his 'EC') and so I had the regular cab selected. Thus, the disparity in values. Yours are the correct ones for an extended cab. (This is precisely the type of thing that causes me to provide the sources of my data when able .... so I can be double-checked and corrected if needed.)

I've got an 06 1500 Z71 w/5.3L and 3.42 gears. Mine is the 5 speed. I hear the 6 speed helps and improves your effective gearing since 1st is lower on yours. So you might want to look at effective gear ratio. I'm towing a 2000lb trailer with 3100lb car on it. Add my crap in the truck. Say 5500lbs minimum.

On open roads flat land it's fine and gets 13mpg or so. When I go up to one of the tracks it's up in the mountains. The truck struggles and complains some. I think it needs a bigger trans cooler and probably 3.73 gears for that. Since I only do it twice a year it's not worth worrying about. I just change the trans fluid more often and use really good synthetic.